OSPF: area range v.s. summary address

This will be a short post, because the topic is rather simple. So... why an entire post dedicated to something as simple as this? Well, because a lot of people are confused by it, and I myself have had an issue a few days ago where I used the "summary" where I should've used an "area X range command".

Basically the difference can easily be summarized :)


Between different Areas of the OSPF (Summarizing the INTRA-AREA routes):

ABR: area [source-area] range [address] [mask]


Redistribution from the other routing protocol (Summarizing the EXTERNAL routes):

ASBR: summary-address [address] [mask]


Thats it, nothing more to it.

SUMMARY ADDRESS

So, for example if we want to configure the summarization on the ASBR, and currently we have:

#show ip route | i E2

      E1 - OSPF external type 1, E2 - OSPF external type 2

O E2     4.4.0.0 [110/20] via 10.1.34.4, 00:01:50, Serial1/0.34

O E2     4.4.1.0 [110/20] via 10.1.34.4, 00:01:50, Serial1/0.34

O E2     4.4.2.0 [110/20] via 10.1.34.4, 00:01:50, Serial1/0.34

O E2     4.4.3.0 [110/20] via 10.1.34.4, 00:01:50, Serial1/0.34

So how can these be summarized? Well, it seems that the /22 mask would do the job, so - lets apply the explained "summary-address" command under the router OSPF process:

(config-router)#summary-address 4.4.0.0 255.255.252.0 

And check the External routes again on the router Cisqueros_R3:

Cisqueros_R3#show ip route | i E2

       E1 - OSPF external type 1, E2 - OSPF external type 2

O E2     4.4.0.0/22 [110/20] via 10.1.34.4, 00:00:01, Serial1/0.34

Also there is a new Static route on the Cisqueros_R4 router pointing to Null0. This is a SUMMARY ROUTE, and it's purpose is preventing Rooting LOOPS:

O        4.4.0.0/22 is a summary, 00:05:58, Null0

There are two types of Summary route:

 1. Internal - when "area x range" summary is configured, the Internal Discard route is created

 2. External - when "summary-address" is configured, the External Discard route is created

If you want the Summary route NOT to be installed in the Routing Table you need to know which of these two types is configured, and apply the "no discard-route" command:

 (config-router)#no discard-route [internal | external]

AREA X RANGE

Just for variety, "area 12 range" command will be applied here on the OSPFv3 (OSPF for IPv6), which has exactly the same appliance as the IPv4 OSPF:
ABR(config-rtr)# area 12 range 2000:111:111:1110::/62

To make sure we´re summarizing correctly, we should first check the LOCAL OSPFv3 Database, and make sure that the summary route is the only one appearing.

ABR#sh ipv ospf data | i 111

2.2.2.2         13          0x80000001  2000:111:111:1110::/62 <- SUMMARY ROUTE

Now check the Routing Table on the OSPF Neighbor in another area:

#sh ipv route ospf | i 2000:111:

OI  2000:111:111:1110::/62 [110/65]